The present invention relates to the field of hydrostatic units used to propel vehicles. More particularly, this invention relates to a neutral override circuit or system for servo controlled hydrostatic units.
A skid steer loader is one type of vehicle that often utilizes one or more hydrostatic transmissions for propulsion. Typically, the vehicle""s engine drives a servo-controlled variable displacement hydraulic pump that fluidly drives a hydraulic motor. The hydraulic motor powers one or more of the vehicle wheels.
Some skid steer loader manufacturers have added service and/or parking brakes to their hydrostatically propelled machines to prevent them from rolling away or being moved inadvertently (inadvertent movement can sometimes occur if the pump of the hydrostatic transmission is accidentally stroked out of its neutral position). In fact, new regulatory requirements are demanding parking brakes on skid steer loaders bound for European markets. Current methods of providing braking systems require the brake to absorb the full torque capability of the hydrostatic transmission. Current braking systems are hard pressed to handle increasing hydrostatic torque capability and machine weight.
One approach to resolving this problem has been to isolate and reduce only the source pressure for the displacement control of the pump. However, this approach requires additional machined passages and valving to separate the control portion of the circuit from the supercharge portion of the circuit.
Therefore, a primary objective of the present invention is a provision of simple neutral override system or circuit that prevents the hydrostatic pump from stroking out of neutral.
Another objective of the present invention is the provision of a neutral override circuit that prevents the hydrostatic transmission from stroking out of neutral yet provides enough charge pressure to lubricate the rotating elements of the pump.
Another objective of the present invention is the provision of a neutral override system that is easy to retrofit to existing hydrostatic transmissions.
Another objective of the present invention is the provision of a neutral override system that is economical to manufacture, as well as durable and reliable in use.
These and other objectives will be apparent from the drawings, as well as from the description and claims which follow.
The present invention relates to the field of hydrostatic units used to propel vehicles. More particularly, this invention relates to a neutral override brake control valve and circuit or system for servo controlled hydrostatic units.
The neutral override circuit of this invention includes a variable displacement pump controlled by a servo fluidly connected to a displacement control valve. The displacement control valve is biased to a neutral position. A charge pump is connected to the pump and the displacement control. A solenoid-operated neutral override brake control valve is connected to the charge circuit upstream of the displacement control valve.
The brake control valve is normally biased to an open position. However, the solenoid urges the brake control valve to a closed position in a response to a signal indicating that the parking brake has been released. In the open position, the neutral override brake control valve dumps the charge flow to the pump case until the charge pressure is inadequate to overcome the centering bias of the displacement control. However, characteristics of the valve can be set so that some minimal charge pressure and lubricating flow are still provided to the rotating elements of the pump.